The present invention relates in general to a multiband antenna, and more specifically to an on-glass automotive antenna having a compact size and performing as a loop antenna at a first digital audio broadcasting (DAB) frequency band and as a quasi-loop antenna at a second DAB frequency band without any switching of antenna elements being required.
Digital audio broadcasting is a broadcast radio service being introduced in many places throughout the world which provides high quality audio and auxiliary data transmissions. One of the most promising applications of DAB is in mobile receivers installed in automotive vehicles, such as cars and trucks.
Various standard transmission protocols, such as Eureka-147, are being established for DAB. European countries and Canada have already begun transmitting terrestrial DAB signals in Eureka-147 format. However, different frequency bands are being designated for DAB service by different governmental authorities around the world. For example, Canadian DAB currently operates in the L-band (from 1452 to 1492 MHz) while European DAB currently operates in Band-III (from 174 to 240 MHz) and L-band.
Depending upon the final decisions that may be taken around the world in selecting frequency bands for DAB systems and depending upon where a particular DAB receiver may be used (e.g., as an automotive vehicle moves between areas), it may be necessary or desirable to receive in both the L-band and Band-III. However, the use of separate antennas on a vehicle for each frequency band is undesirable because of cost, appearance, and space limitations.
Vertical monopole whip antennas are known which can provide reception in both L-band and Band-III. Whip antennas, however, are undesirable because they create wind noise, are an unattractive protrusion, and are subject to breakage.
Conformal antennas, carried by a vehicle surface such as a window glass, are preferred for automotive vehicles for improved appearance, durability, and elimination of wind noise. However, there are several complications in attempting to design a conformal antenna which is capable of receiving terrestrial signals in both L-band and Band-III. The difficulty results, in part, from the fact that L-band and Band-III are relatively far apart from each other in frequency. Another source of difficulty is the limited space available on a window glass.
Since terrestrial broadcast signals become vertically polarized, one might consider the approach of forming a vertical quarter-wave monopole antenna on a vehicle window to receive both frequency bands. However, the vertical length for such an antenna receiving Band-III is about 350 mm. Therefore, the vertical antenna conductor would mechanically interfere with window-mounted heater wires for the window defogger which are widely used on rear windows. Placing the antenna on the front window where more space is mechanically available is undesirable because the antenna would impinge in the direct, forward-looking field of vision.
Related U.S. application Ser. No. 08/841,315, filed Apr. 30, 1997, entitled "Multiband Reception Antenna for Terrestrial Digital Audio Broadcast Bands", which is incorporated herein by reference, teaches an antenna structure that includes a loop portion for receiving at Band-III frequencies and a half-wave dipole portion for receiving at L-band frequencies. Impedance circuits in the loop portion isolate the loop portion at L-band frequencies. However, antenna gain at L-band frequencies may be too low for some applications due to reduced current flow caused by a junction within the dipole antenna portion.